The rearrangements of ethynamine 3 (H-CºC-NH2) to ketenimine 4 (CH2=C=NH) and acetonitrile 5 (CH3CN) were investigated computationally up to the MP4(SDTQ)/6-31G*//MP2(FU)/6-31G* level. The calculated barrier for a concerted reaction 3 -> 4 is very high, 74 kcal/mol, the structure of the transition state very unusual, and this path is discredited. A lower barrier of about 60 kcal/mol via aminovinylidene 2 and imidoylcarbene 15 has been found. The calculated barrier for a concerted second step 4 -> 5 is 61 kcal/mol, and the transition state structure is again very unusual with a virtually linear CCN backbone, but this does not appear to correspond to physical reality. Instead, CASPT2 calculations predict reaction via vinylnitrene 9 and/or homolysis of 4 to the radical pair ·CH2CN + H· (11) with a barrier of 67-70 kcal/mol in agreement with experimental shock-tube data. Recombination (maybe via roaming) affords acetonitrile 5. There is strong experimental evidence for homolytic paths in pas-phase pyrolyses of ketenimines.
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